Investigation of low lying electric and magnetic dipole excitations in heavy nuclei

NUCLEAR PHYSICS A

Nuclear Physics A577 (1994) 191c-196c North-Holland, Amsterdam

Investigation of low lying Electric and Magnetic Dipole Excltatmns m heavy nuclei * P von Brentano a, A Zilges% R - D H H Pitz b

Herzberg% U Kneissl b j Margrafb, and

a Institut fur Kernphyslk, Universitat zu Koln, D-50937 Koln, G e r m a n y b Instatut fur Strahlenphysik, Universitat S t u t t g a r t , D-70569 S t u t t g a r t , G e r m a n y A s u m m a r y of new results for the systematics of dipole excitations in the rare earth region is given It is shown that the absolute strengths of the the M1 Scissors Mode is well reproduced by a sum rule approach In addition strong electric dipole transition to low lying 1- states have been observed in spherical as well as in deformed nuclei They can be described by an octupole excitation mechanism 1. I N T R O D U C T I O N Throughout the last decade the combined analysis of electron, photon, and proton scattering d a t a gave new insights into the structure of low lying dipole excitations in nuclei In 1983 the D a r m s t a d t group around A Richter discovered a collective, lsovector magnetic dipole excitation usually denoted as the "Scissors Mode" at an energy of ~_ 3 MeV In the strongly deformed nucleus l~6Gd in (e,e')-experiments [1] Later the existence of this new M1 excitation mode was systelnatic,dly Investigated in a large number of isotopes in electron as well as in photon scattering expermments [2-4] In the first part of this contribution we will discuss the results of the (7, 7 ~) experiments on rare earth nuclei in the context of a simple M1 sum rule The second part of the paper deals with electric dipole excitations The l - octupole excitations observed in different heavy nuclei exhibit B(E1) strengths to the groundstate which are up to three orders of m a g n i t u d e larger than the average B(E1) values [5] The systematics of the experimental E1 strengths distributions are given 2

MAGNETIC

DIPOLE

EXCITATIONS

The discovery of the Scissors Mode ten years ago by the D a r m s t a d t group started a large n u m b e r of e x p e r i m e n t a l and theoretical studms From the (% 7 ~) d a t a collected in *This work was supported by the Deutsche Forschungsgemelnschaft under contracts Br 799-33/34 and Kn 154-21 and partially by the DFG-JSPS cooperation agreement 0375-9474/94/$07 00 © 1994 - Elsevier Science B V All rights reserved SSDI 0375-9474(94)00370-X

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P von Brentano et al / E l e c t r t c and magnettc dtpole excttattons

the last years the total strength systematics in the nuclei of the rare earth region can be obtained Figure 1 displays the summed B(M1)T strengths between 2 6 and 4 MeV m nuclei of the mass region A=142 to 186 examined by the S t u t t g a r t - D a r m s t a d t - G i e s s e n Cologne collaboration [6-12] One can see that the M1 strength increases with increasing mass number up to A'-~154, then it stays nearly constant before it drops again for the heawer nuclei

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Mass Number A Figure 1 The sumined B(M1)T strengths between 2 6 and 4 MeV m the rare earth region The figure includes the d a t a points for transitional as well as for strongly deformed nuclei The i64Dy point was corrected by taking out the spin part

The d a t a on the detailed M1 strength distribution and on the s u m m e d B(M1) strengths are a challenge to the v i n o u s theoretical models for the scissors mode [13-19] Recently some interesting proposals on sum rules have been m a d e [20,21] In particular N Lo Iudlce and A Richter [22] derived such a sum rule for the scissors mode starting from the sum rule by E Lipparm] and S Stringan [23]

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P von Brentano et al / Electrzc and magnetzc &pole excttattons

193c

strength given by A Bohr and B R Mottelson [24] from which one can derive Os . . . . . and finally gets the sum rule for the M1 strength of the Scmsors Mode 4NZ

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The maan varmtlon of the B(M1) strength m a certain mass region results from the change of the square of the deformation, 62 The 6Ldependence of the Scissors Mode strengths has been dascovered In experiments by W Ziegler et al for the Sm ]sotopac chain [25] and shown to hold also in the Nd isotopes by J Margraf et al [26] Figure 2 shows the quahty of the sum rule (eq 2) for all hatherto examined nuclei of the rare earth region It gives the ratm of the experimental and theoretical strength versus the mass number A The overall agreement as very good, the sunple formula works for the transitional nuclei as well as for the strongly deformed nuclei The absolute B(M1) strength is predicted an this sum rule w,thout any free parameter Nevertheless one should note that th]s sum rule as model dependent

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Figure 2 Rat]o of the summed experamental B(MI)T strength an the interval of 2 6-4 MeV derived from (7,'7')-experiments and tile theoretical predlctmn of the sum rule derived by N Lo Iudlce and A Rachter [22] We took a constant value ~os...... =3 MeV

The experlmentM progress of the photon scattering experunents now permits the study of the Scissors Mode in odd A nuclei also Thus recently the existence of the Scissors Mode m an odd A nucleus was discovered in an (7, 7')-experiment on 163Dy [27] The summed B(M1)T strength m 163Dy between 2 6 and 4 MeV as B(M1)T=0 9+0 2#~v a calculataon m the Interacting Boson-Fermmn Model (IBFM) was able to reproduce the main features of this excitation [27] In contrast to the odd N nucleus 163Dy no M1 strength has been

194c

P von Brentano et al /Electrtc and magnettc dtpole excttattons

observed in the odd Z nucleus 165Ho [28] This is not understood and mdacates that further experiments on &fferent odd nuclei are urgently needed to u n d e r s t a n d this phenomenon In combination with theoretical studies they should lead to a deeper insight into the structure of the Scissors Mode 3. E L E C T R I C

DIPOLE

EXCITATIONS

In m a n y nuclei a low lying 3 - excltahon is found which is usually interpreted as a octupole shape v l b r a h o n In spherical nuclei this octupole vibration can couple to the quadrupole vibration leading to a multlplet of two phonon states with spins and parities J ' = l - - 5 - The centrold energy of the multlplet should be near the sum energy of the smgle wbratlons, i e around 3 5 MeV m the N=82 region The deexcitatlon of the 1- state of tins multlplet to the groundstate can be observed as a strong electric dipole transition in photon scattering experlinents In the leftmost colunm of figure 3 one can see that m the N=82 nuclei 14°Ce and 142Nd which were recently studaed by us only one 1- state was Identified m the energy region between 1 and 4 MeV The B(E1) strengths are two to three orders of m a g m t u d e larger than the aver,~ge values m this mass region [29]

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Energy [MeV] Figure 3 Distribution of electric dipole strength In different rare earth nuclex For the strengths we give the parity independent value c F~~d, where F~~d -- Fo/(Ea~(MeV) -3) and where c = 2 866x10 -3 e 2 f m 2 ( m e V ) -1 We note that for electric dipole transltaons c F ~ = B ( E 1 , 0 + --* 1 - ) Please note that 14°Ce and 142Nd are spherical nuclea whereas the other nuclei are well deformed [5]

In well deformed nuclei tile octupole vibration couples to the quadrupole deformed

P von Brentano et al / Electrw and magnettc &pole excttattons

195c

core leading to octupole vibrational bands which can be characterized by their K quantum number The bandheads with J = l can be populated in photon scattering via E1 transitions from the groundstate It has been shown that in general E1 transitions with A K = 0 are much stronger than those with A K = I [30] Therefore we expect to excite in our strength selective photon scattering experiments usually only 1- states with K=0 in the energy region around 1-2 MeV Figure 3 shows that this indeed is the case in nearly all examined well deformed isotopes 14s'lS°Nd, lss'lSS'l~°Gd, 16°'162'164Dy, 16~'16s'lT°Er, and 172'174yb It Is interesting to note that the B(E1) strengths of these lowest lying octupole states in the well deformed nuclei are comparable to the strengths of the two phonon 2 + ® 3- states in spherical nuclei This indicates that there is a common excitation mechanism (the octupole vibration coupled to the quadrupole vibration or to the quadrupole deformed core) for the lowest 1- states both in spherical as in deformed nuclei The investigation of Multi Phonon Excitations opens a wide field of research actlvltms Recently 2+ ® 3- ® particle excitations have been identified in (7, 7') experiments on the odd N nucleus 143Nd [31] This discovery started a number of experimental and theoretical studies in the N=82 region In the well deformed nuclei the analysis of photon scattering experiments using the new Compton polarlmeter setup gave first evidence for the coupling of octupole and 7-vibration [32] The high precision and selective data obtained with electromagnetic probes are promising a lot of new Interesting information about the structure of electric dipole excitations Acknowledgements Most of the presented results have been obtained in a very close collaboration of the universities Stuttgart, Glessen and Cologne We thank our colleagues from this collaboration, especially I Bauske, R D Hell, H Maser, N Pletralla, and C Wesselborg for their work The authors gratefully acknowledge valuable discussions with A Arlma, R F Casten, F Donau, A Faessler, A Gelberg, I Hamamoto, K Heyde, R V Jolos, R Nojarov, T Otsuka, A Richter, J Speth, K Sugawara-Tanabe, H J Wortche, N V Zamfir, and W Zlegler REFERENCES

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